In the petroleum refining industry it is often useful to upgrade certain oil and fractions like heavy oils and residuum by hydrotreating. Examples of such hydrotreating processes are hydrodemetallation, desulfurization, and denitrogenation. In these processes the feedstock is contacted with a hydroconversion catalyst in the presence of hydrogen at elevated pressure and temperature. Due to strict demands imposed by ecological regulations, the refining industry has become increasingly more focused on producing high quality cleaner fuels with a minimum content of contaminants such as sulfur, nitrogen and heavy metals.
Catalysts used in hydrotreating processes generally comprise catalytically active metals from Groups 6, 9 and 10 of The Periodic Table and are typically supported on alumina, which may be combined with other inorganic refractory materials such as silica, magnesia, titania, zirconia and the like. Secondary promoters or additives, such as halogens, phosphorous and boron, have also been used to enhance catalytic properties. To achieve the maximum effect from hydrotreating processes, it is necessary to optimize catalyst activity and selectivity to a desired hydrotreating reaction. Catalyst activity and selectivity are determined and affected by such factors as the nature and properties of the catalyst support, the catalytic agents, activity and selectivity of promoters as well as the preparation and activation method used.
Where heavy feedstocks contain organometallic compounds, the effectiveness of the hydrotreating as well as downstream catalysts tend to decline relatively rapidly, particularly when the impurity is more than about 10 to 20 ppm metals such as dissolved nickel and vanadium. These metallic impurities are said to deposit on the surface and in the pores of these catalysts reducing their effectiveness. One approach to the problem of metal impurity has been to alter the pore structure of the hydrotreating catalyst. However, the determination as to which pore structure to use is unpredictable and not easily obtained. There is yet a conflict in the art regarding optimal pore structure. Several patents which have discussed this conflict include U.S. Pat. No. 4,066,574; U.S. Pat. No. 4,113,661 and U.S. Pat. No. 4,341,625.
Hydrotreated hydrocarbon feedstocks having a low Conradson carbon residue are also highly desirable in the refining industry. Carbon residue is a measurement of the tendency of a hydrocarbon to form coke. Expressed in weight percent, carbon residue may be measured as microcarbon residue (MCR). The MCR content in a hydrotreated residual feedstock is an important parameter since the hydrotreated residue usually acts as feed to a coker or a fluid catalytic cracking (FCC) unit. Decreasing the MCR content in the hydrotreated residue decreases the amount of low value coke generated in the coker and increases the amount of gasoline generated in the FCC unit.
To this end, there remains a need to develop catalyst compositions that are less expensive and/or more effective in removing metal contaminants from hydrocarbons feed streams, in particularly heavy hydrocarbon feed streams, than catalysts presently employed. There also remains a need for improved hydrodemetallation and/or hydrodesulfurization catalysts which provide good MCR conversion during a hydrotreating process.